In the process of manufacturing electronic printed circuit boards (PCB) and similar articles, such as Multi-Chip-Modules (MCM) and Flat Panel Displays(FPD), a layer requiring selective etching needs a protective coating from the non-etched areas. The best known resists are photoresists, widely used in the PCB, MCB, FPD and semiconductor industries. Selected areas of the resist have to be removed in order to allow the etching process. After etching, the remaining resist is removed from the protected areas, in a process known as "stripping". The common way of selectively removing the resist is by changing its solubility, via selective exposure. The exposure can be done via a mask (contact or projection printing) or by direct writing using a laser. The current invention is most suitable to the direct writing with an IR laser. Depending on the type of resist used (positive or negative), the laser exposed area becomes more soluble or less soluble in a liquid known as a "developer". After developing, the soluble areas of the resist are removed and the article can be selectively etched. In this context the current invention is a "negative acting thermoresist".
A chemical composition capable of switching from a water soluble or water washable state to an insoluble state when heated, preferably by a focused IR laser, has been discovered. The degree of solubility can be controlled over a wide range by mixing the composition with different polymers. Such compositions are of greater commercial importance in the manufacturing of printed circuit boards, replacing the current UV exposed photoresist with IR exposed thermosensitive resists. The IR exposure allows faster and environmentally cleaner imaging as development can be done with plain water. In this disclosure the terms "water soluble" and "water washable" are used interchangeably to describe the ability of the unexposed polymer to be washed away by water, regardless of the exact nature of the action of water or water base solution on the polymer.
Prior art thermosensitive resists required a multi-step or multi-layer process which was not cost effective. The process of manufacturing printed circuit boards using a single layer thermoresist is covered by a co-assigned application, 08/653,901,now abandoned.